Kinetic and Thermodynamic Investigations of CO2 Insertion Reactions into Ru–Me and Ru–H Bonds – An Experimental and Computational Study

The rates of CO2 insertion into trans-Ru(dmpe)2(Me)H [1, dmpe = 1,2-bis(dimethylphosphino)ethane] and trans-Ru(dmpe)2(Me)2 (2) derivatives were monitored by in situ infrared and 1H NMR spectroscopy. The reactions are first order in both CO2 and metal complex concentrations, and CO2 insertion into the Ru–H bond of 1 occurs instantaneously at 0 °C. The reverse process, decarboxylation, was observed to occur readily at ambient temperature as revealed by 13CO2 exchange with subsequent CO2 insertion into the Ru–Me bond at higher temperatures. No further CO2 insertion into the Ru–H bond of the resulting acetate complex was observed. The activation barrier for CO2 insertion into the first Ru–Me bond of 2 was determined to have ΔH‡ and ΔS‡ values of 12.7 ± 0.6 kcal mol–1 and –31.9 ± 2.0 e.u., respectively, which are indicative of a highly ordered transition state. The rate of CO2 insertion into the second Ru–Me bond was two orders of magnitude slower at ambient temperature and resulted in the formation of trans-Ru(dmpe)2(O2CMe)2. In general, the insertion of CO2 into the Ru–H or Ru–Me bonds of trans-Ru(dmpe)2(X)R (R = H or Me) was disvavored in the presence of poorly electron-donating X ligands. For example, the insertion of CO2 into the Ru–H bond of trans-Ru(dmpe)2(Cl)H was not observed even under forcing conditions. Computational results were in excellent agreement with these observations and predict a significant enhancement in CO2 activity and resultant complex stability if dmpe is replaced with tetramethylethylenediamine (tmeda).